Acoustic device



Nov. 17, 192 1,562,165

H.C.HARR$ON v ACOUSTIC DEVICE' I FiledjJuly 17. 1925 v A 00 6 arr/L50,

Patented Nov. 17, 1925.

UNITED STATES PATENT OF-FlCE.

HENRY C. HARRISON, OF PORT WASHINGTON, NEW .YORK, ASSIGNOR TO WESTERNELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y.,.A CORPORATION OF NEWYORK.

AcoUs'rIc' nnvron.

Application filed July 17, 1923. Serial No. 652,013.

To all whom it may concern:

Be it known that I, HENRY C'. HARRISON, a citizen of the United States,residing at Port Washington, in the'county of Nassau, State of'New York,have invented certain new and useful Improvements in Acoustic Devices,of which the following is a full,

clear, concise, and exact description.

This invention relates to acoustic devices, the object being to providean acoustic device capable of faithfully and efficiently reproducing allvibrations in the sound range I and particularly the vibrations of thelower frequencies. I

In general, the inventionprovidesan electro-magnetic device having avibrating system, with restoring means capable of exert ing a forcewhich varies with the displacement of the vibrating member insubstantially the same manner as the magnetic pull of the steady fieldon the vibrating member varies.

'In one embodiment the invention contemplates a loud speaking receiverof the bal-; anced armature type, in which the armature the magneticfield.

Referring to the drawings:

Fig. lis a view, part in section, of an electromagnetic system for loudspeaking receivers embodying the invention;

' Fig. 2 is a cross-sectional view through the armature supporting.means;

- Fig. 3 is a sectional view taken on line 3'-3 of Fig. 1;

Fig. 4 shows comparative curves of operation of present receivers andthose embodying theinvention, Y

Fig. 5 shows a loud speaking receiver embodying the invention in amodified form; and

Fig. 6 is a view through section 66 of cies below the resonant frequencyof a receiver, or of a :similar electromagnetic re-' producing devic areusually poorly reproduced because 0 the high elastic reactance small andthe natural frequency cannot be.

brought 'down to a low audible frequency.

Referring'to the curves-shown in Fig. 4:, A-represents a typical forceversus displacement curve for the steady field of a receiver and B theforcewersus displacement'curve of -a restoring elasticity, such as .areed or a spring supported armature. From the curves A and B it isevident that the force shown bycilrve B atzero separation must begreater than that shown by curve A to prevent freezing of the armatureto the pole pieces. armature,- and which determines its naturalfrequency is the difference between the elasticities shown by curves Aand B and is represented by the slope of curve F. A vibratingsystemhaving anelasticity curve of this character produces distortion inreproductions due to the addition'of harmonics.

. To. avoid these'distortions' it is necessary shown by curve D, or toapply a restoring force having characteristics shown-by curve C, whichis a curve of the same order as curve A. Curve 0 is shown plotted on theoppoiste'side of the horizontal axis for the reason that this forceopposes that one to the steady magnetic field. For small separationsbetween the armature and the pole pieces of the receiver, the pull onthe armature due to the magnetic field varies approximately inversely asthe square of the separation. Both curves, A and O, are thereforeapproximately parabolas. The first solution shown by curves B and D inThe net elasticity operative on the volve in general a loss inefficiency asit suitable permanen't. magnet.

would require a substantially large separa tion between the pole piecesto change the magnetic field to have the same characteristies as shownby curve D. Therefore, for the purpose of describing the invention, thestructures which have the characteristics shown by curves A and C willbe described.

In the embodiment shown in Figs. 1,2. and 3, a core "11, having two arms12 and 13-}, carries a coil 14, which when connected toa source ofelectrical energy furnishes a steadymagnetic field. If preferred, thecoil 14 and the-core ll may be replaced'by a The core 11 may besupported on a case 15, (only part of which-is shown and which may be ofany suitable form) by the studs 16 and the suitable means in the case15. The arms 24 1 and 25 of the support 22 are provided with recesses 26and 27 respectively, and the middle arm 28 with a recess 29. Thearmature 23 extends through the recesses inthese arms and is supportedon the arms 24 and 25 by means of resilient. members or springs 30 and31, respectively. These springs are short "and so designed as to offer,a resistance to the novement of the armature 23, which var es insubstantially the same manner as the'magnetic pull on the armature. Inthis case this resistance varies as-sho-Wn by curve l" or magneticsystems having.

C in Fig. 4-. a maximum movement of .014; inches, springs having athickness of.()tl2 inche'sa-nd an unsupported length of about .060inches have i been found. satisfactory. .The armature 23 with respect tothe poles.

may be supported intermediate its QXtl'ClIb ities by any suitable means,such as a pin 3 one end of which fits loosely in the armaturc 223 sothere will be no initial stress on the springs 3.0 and 31 and the otherend may tit snugly in the-arm 28. This pin has pre erably a reducedmiddle section so as to add no stiffness t0 the vibratin system. Itsfunction is to provide a definite center of oscillation for the armatureand may be omitted if the springs 30 and 31 are exactly the same anddisposed exactly symmetrical sociated with the armature 23 -hy means ofrod 32 is a diaphragm -83 of the usual type,

'which operates" into a horn orid istributes the sound in any wellknown-'Inanner. The rod 32 is preferably adjusted so there willOperatively as.-

' brating systems.

"be no'stress exerted on the armature by the v I diaphragm at maximumseparation.

It will be noted from the curves shown the magnetic field, thearmaturewould have zero elasticity and zero natural period for alldisplacements and would therefore remain infany position in the magneticfield to which-it would bemoved. As the armature would freeze under thiscondition for zero separation, it is preferable to provide a restoringforce greater than the magnetic pull by an amount proportional tothedis-= placement. One having a low constant elasticity of such a value asto give the vibrating system a low natural frequency of about 50 to'200cycles per second has been found very satisfactory. This willnot onlypre vent freezing at zero separation but will make the device slightlymore efficient at the lower frequency range where it is de-.

sirable. For very slight deflections of thesprings 30 and 31 therestoring force is quite small, but as the deflection is increased theforce increases rapidly and at substantially the same rate as the pulldue to the steady field. A I

The modification shown'inFigs. 5 and 6 only in the manner of supportingthe armat'ure. the support 22 in the previous modification, hasa convexbearing surface 41 on its middiffer from the one hereinbefore describedA support 40, which'corresponds to i (lle arm, against which thearmature, 42 is held by a resilient member 43 having aknifeedge 44engaging a groovel45 in the armature/12, the bearing surface 41 shouldhave 'a'large radius. For the structure shown a 2" radius has been foundto be satisfactory. As the armature 42 vibrates, the distance betweenthe bearing point on the member. 43 and the varies. e 4

The restoring force varies, with this )ivotin' oint on the surface 1-1"1 s p iii) change in distance and is substahtially proportional thereto.The restoring force also changes with the pressure exerted on thearmature by the member 43 which is flexible for vibrations of smallamplitudes and acts as a variable load; As the change in the distance.between the supports and the change in the load are both in the samedirection the force displacement curve may be made to increase at such arateas to substantially equal that due to the steady magnetic field orany desired part of it at transmitters, recorders, reproducers or anyelectromagnetic apparatus employing vi- The invention claimed is: 4

1. In a sound reproducing system, a diaphragm, a magnet, an armature forsaid magnet operatively connected to the dia- 5-phragm, and means forsubstantially bal- 3. In an acoustic device, a magnet having asubstantially constant magnetic field, a vibrating member disposed inthe field of said magnet, and means for balancing the pull of themagnetic field on said member,

said means having a stifiness such that itsrestoring force is slightlygreater than the magnetic pull for substantiallyall positions of thevibrating member by an amount proportional to the displacement of thevibrating member from its neutral position.

4. In an acoustic device, a magnet having a substantially constantfield, an armature disposed in said field, and resilient supports .forsaid armature, said supports producing a restoring force which varieswith the minute displacement of said armature in substantially the samemanner as the magnetic pullon said armature but in the oppositedirection.

5. In a magnetic system, a magnet, a vibrating member operativelyassociated with said magnet, and means for offering a me chan1calimpedance to the armature which varies in the same manner as themechanical impedance due to the force of the steady magnetic pullexerted thereon.

6. In an acoustic device, a diaphragm having a substantially steadymagnetic field, an armature connected to said diaphragm and mechanicallyassociated with said magnet, and means for balancing the armatureagainst the pull of said magnet, said means being of such stifi'nessasto cause a resisting force which varies with the displacement of saidarmature in substantially the same manner as the pull of the magneticfield. I

7'. In a sound reproducing mechanism, a diaphragm, a magnet having asteady mag netic field, an armature for said magnet responsive tocurrent variations operatively.

connected with the diaphragm, and means offering an impedance to themovement of. the armature substantially op osite in phase.

and equal in magnitude to t e mechanical impedance due to the steadymagnetic field 8. In a sound re roducing mechanism, an electromagnet proucing a steady magnetic field, an armature, a diaphragm operativelyconnected with said armature, and means for producinga force in thearmature resistto the diaphragm, and a spring connected to netic fluxvaries with the displacement of ing the magnetic force of thesteadyjfield 55 and larger by an amount proportional to thefdisplacementof the armature for substantially all positions.

9. In a'sound reproducing devioe',l' 'g diaphragm, an electromagnetresponsive'to'f the current variations, an armature. fortheelectromagnet pivotally supported between its ends and operativelyconnected at one end to the diaphragm, and damping means connected' withthe other end of said armature adapted to resist the movement of the.arma I ture in response to the magnetic force thereon and in directproportion to the magnetic 'force.

10. In a sound reproducing device, a diaphragm, an electromagnetresponsiveto current variations, an armature for theelectromagnet'pivotally supported between its endsand operativelyconnected at one end the other end of said armature and so proportionedwhen mounted that its resistive'force to the movement of the armature inresponse to the magnetic force is in direct proportion to such force.

11. The method of controlling a vibrating system which consists inapplying a steady magnetic field and an opposing resisting force to avibrating member, the resisting force varying with the displacement ofthe member in a manner such that thedifierence between the resistingforce and the magnetic pull gives a constant elasticity.

12. The method of controlling a vibrating member which consists inapplying thereto a steady magnetic field and an opposing resisting forcewhich varies with the displacement of the vibrating member in the samemanner as the pull due to the magnetic field varies. 3 i

13. The method of controlling thegvibrating system of a magnetic devicewhich'consists in sustantially counter-balancing the elasticity in' themagnetic field by a second elasticity varying with the displacement ofthe armature in the same manner as the elasticity of the magnetic field.

14.,The method of controlling the vibrating system of a magnetic devicewhich consists in applying thereto a restoring force which varies withthe displacement of the vibrating system in substantially the samemanner as the force due to the steady magthe vibrating system. no

15. The method of controlling a vibrating member in' a magnetic systemwhich consists in applying 'a resisting forceto the vibratingmemb'erwhich varies substantially in-, ver'sely as the square of thedisplacement'of the vibrating member. *1

16. In a vibratory electromagnetic system responsive to frequencieswithin the voice frequencies.

range, a magnet, a vibrating armature for range,' a magnet, a vibratingarmature for said magnet, and resilient means for plae 1 said magnet,and resilient means for making ing the-"natural frequeney of vibrationof the arnlature'highly responsive to the lovver 0 the armature inithelower range of voice range of voice-frequencies,

, I v In witness whereof, I hereunto subscribe 17." In a Vibratoryelectromagnetic system"1ny name'th-is 11 day of July A. D.,' 1923.-responsive td fre'queneies Within the voice HENRY C. HARRISON.

